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Build an 8-cha stereo mixer; Building the new 8-Channel Stereo Mixer is straightforward since nearly all the parts are mounted on one large PC board. A second, much smaller board takes care of the power supply components. By JOHN CLARKE 80  Silicon Chip The general arrangement of the new mixer can be quickly gleaned from the accompanying photos. As shown, the main board mounts on the back of the front panel, while the power supply board mounts on the base of the case. The main board is coded 01210961 and measures 400 x 290mm, while the power supply board is coded 01210962 and measures 105 x 67mm. Begin construction by checking the PC boards for shorted or broken tracks annel Pt.2 and by checking the hole sizes. All holes for the pots should be just large enough to accept the threaded section, while the holes for the 6.35mm stereo sockets should be 11mm in dia­meter (to accept the stub at the end of each socket). Rotary switch S9 is mounted directly on the main board and should be test fitted to ensure that its mounting holes are large enough to accept the switch pins. The eight toggle switches (S1-S8), the 6.35mm socket contact points and the three power supply inputs are all soldered to PC stakes. Check that the relevant holes are large enough to accept the PC stakes supplied. The holes for the XLR sockets and plugs should also be tested for correct size. In addition, there should be a 12mm hole directly below S9 and in line with the Effects pots. This hole allows the relevant leads to pass through the PC board on their way to the mains switch (S10) and to an adjacent earth solder lug bolted to the front panel. Finally, there are 11 mounting holes for attaching the PC board to the front panel (via 3mm screws and 25mm standoffs). Check that all these holes are drilled to 3mm. Fig.4 shows the assembly details for the main PC board. Begin by inserting the PC stakes, wire links and resistors. The PC stakes are installed at the S1-S8 switch positions (three for each switch), at the 6.35mm socket connection points (tip, ring and ground), and at the +15V, GND and -15V power supply points located near LED1. Table 2 shows the resistor colour codes but we also suggest that you check each value using a digital multimeter, just to make sure. That’s because some colours can be difficult to deci­pher and it’s easy to make a mistake when there are lots of resistors to be installed. The next step is to install the ICs. Be sure to install the correct op amp in each location and with the correct orientation. In particular, note that IC6 and IC8 are oriented differently to the remaining ICs. Once the ICs are in, install the two signal diodes (D1 and D2) and transistors Q1 and Q2. Don’t get these two transis­tors mixed up – Q1 is a BC338 NPN type, while Q2 is a BC328 PNP type. The capacitors are next. Note that the electrolytics with 10µF and 100µF markings are polarised and must be oriented as shown on Fig.4. The remaining electrolytics are NP (nonpolar­ised) and can be oriented either way around. Table 1 shows the codes used for the ceramic and polyester capacitors. It’s now time to install the switches. The toggle switches (S1-S8) are simply soldered to the tops of their corresponding PC stakes. Take care to ensure that each switch is centred over its PC stakes and that it is at right angles to the PC board before soldering all the connections. Rotary switch S9 is mounted directly on the PC board. Push this switch all the way down onto the board before soldering its pins. This done, remove and discard the locking ring that’s located under the mounting nut and star washer. The 6.35mm sockets are mounted with their rear stubs in­serted into the PC board holes. Note that it is necessary to bend the Tip (end terminal) outwards and the GND terminal (front) inwards so that they contact their corresponding PC stakes. The Ring (centre terminal) is left unchanged. The Tip and Ring terminals can now be soldered directly to the Tip and Ring PC stakes. The connection from the GND terminal to the GND PC stake requires a short length of tinned copper wire to bridge the gap. Note that only the Tip and GND terminals are used in some cases, and the Ring terminal is left unconnected. For the headphone socket, however, the Tip and Ring terminals are connected together, so that the sound is fed to both sides of the head­phones. Potentiometers There are 54 potentiometers to be mounted on the board, so installing them all will take some time. The main thing to watch out for here is that their values differ, so be sure to install the correct pot at each location. As shown, the pots are all mounted from the underside of the PC board. Before doing this however, it is necessary to fit a 10.5mm ID x 2mm thick plastic washer to the threaded bush of each pot. This is to prevent the pot bodies from shorting the copper tracks on the PC board. In addition, the locating tag on the side of each pot must be bent sideways (to clear the PC board), while the centre terminal must be bent at right angles, towards the shaft. This done, secure each pot to the PC board in turn and solder its centre terminal directly to its PC board pad. The outer pot terminals are wired using short lengths of tinned copper wire. Once all the pots are in, trim the plastic shaft of switch S9 to the same height as the pot shafts. The XLR sockets and plugs can now all be soldered in place, making December 1996  81 82  Silicon Chip December 1996  83 Fig.4: follow the order suggested in the text when installing the parts on the PC board and note that the pots are fitted with an insulating washer (see text) and installed from the copper side. Take care to ensure that all polarised parts are correctly oriented and that the IC type numbers are correct. Repeated from last month’s issue, this photo shows the fully-assembled PC board. Note that a few changes were made to the board after this photo was taken and so it will differ slightly from the layout shown in Fig.4. sure that they sit square with the PC board. This done, insert the two LED bar­ graphs. Note that, in each case, the anode (longer) lead goes towards switch S9. Similarly, install LED21 but do not solder this LED or the bargraphs just yet; first, we have to temporarily fit the front panel. The front panel is straightforward to fit. Begin by fitting the 25mm spacers to the PC board,. There are 11 spacers in all and these should be firmly secured using 3mm machine screws. Once the spacers are in place, fit the front panel over the top of the PC board, so that it sits on the standoffs. The toggle switches (S1-S8) and 6.35mm socket securing threads should protrude through the panel, although you may need to slightly adjust the toggle switches for correct alignment. Check also that the XLR sockets and plugs sit flush with the underside of the panel. When everything fits correctly, secure the 84  Silicon Chip panel from the top using 3mm screws into the spacers. The LED bargraph displays and LED21 can now be pushed into their respective holes in the front panel and their leads sol­dered. This done, remove the front panel and fit the mains toggle switch (S10). In addition, an earth solder lug should be securely bolted to the back of the front panel, immediately to the left of the mains switch. Be sure to scrape away the paint around the mounting point to ensure a good earth contact. Power supply It’s now time to build the power supply and this can begin with the PC board assembly – see Fig.5. The four diodes (D3-D6) can go in first, followed by PC stakes at the six external wiring points. The two 1000µF electrolytic capacitors are mounted side-on, which means that the leads must be bent at right angles to go through the PC board. Take care to ensure that they and the smaller 10µF capacitors are mounted with the correct polarity. It’s a good idea to glue the bodies of the 1000µF capacitors to the PC board, so that they cannot move and place stress on their leads. The two 3-terminal regulators are fitted with small heat­ sinks and the assemblies secured to the PC board using 3mm screws and nuts. There’s no need to isolate their tabs from the heat­sinks but note that REG1 is a 7815 while REG2 is a 7915, so don’t get them mixed up. Bend the regulator leads at right angles so that they go through the holes in the board and trim off any excess after soldering. The remaining power supply items are mounted on the chassis – see Fig.6. If you are making your own metalwork, you will need to drill holes for the fuseholder, cordgrip grommet, transformer mounting screw, earth screw and the mains terminal block. Four mounting holes are also required for the power supply board. The board can then be mounted on 6mm standoffs and the other major hardware items installed. Note that the earth solder lugs must be securely bolted to the case using a screw, nut and starwasher. A second nut can then be used to lock the first, so that there is no chance of it coming loose. As before, scrape away the paint from around the mounting hole before installing this assembly, to ensure a good earth contact. Follow the layout shown in Fig.6 exactly when installing the mains wiring. The mains cord enters the case through the cordgrip grommet and must be securely clamped (check this care­fully). The Neutral (blue) lead goes directly to the main termi­nal block, while the Active (brown) lead goes to the fuseholder. A mains-rated lead is then run from the other terminal on the fuseholder to the terminal block. Slip a short length of heatshrink tubing over the leads before soldering them to the fuseholder. This done, solder the leads, then push the heatshrink tubing over the fuseholder and shrink it down using a hot-air gun. Do not neglect this step – it is an important safety measure to prevent accidental contact with the mains. The Earth lead (green/yellow) from the mains cord must be securely soldered to the earth lug. Leave this lead longer than the others, so that it will be the last lead to break should the mains cord ever come adrift due to some brute force. After making the Fig.5: this is the parts layout for the power supply PC board. Use PC stakes at the external wiring connections and make sure that you don’t get the positive and negative 3-terminal regulators mixed up connection, use a multimeter to confirm that there is zero ohms resistance between the earth pin of the mains plug and the case. The remaining connections to the terminal block involve the transformer primary leads plus a 3-wire mains-rated cable that runs to switch S10 on the front panel and to the adjacent earth solder lug. Note the .001µF 2kV ca­pacitor that’s connected to the terminal block in parallel with these leads. The leads are fed through the 12mm hole in the PC board below S9, before being connected to the power switch. As with the fuseholder, be sure the shroud the switch body with heat­ shrink tubing – ie, slip heatshrink tubing over the leads before soldering them to the switch terminals, then Below: the view from the rear of the PC board, with all the pots in place. The two outer terminals of each pot are soldered directly to the copper pads, while the centre terminals are connected via a wire link. December 1996  85 Fig.6: install the power supply wiring as shown in this diagram. Note that the leads to power switch S10 must be mains rated, as must the Earth lead to the adjacent solder lug. push the tubing over the switch body and shrink it down. The transformer secondary leads can now be connected to the power supply board but don’t connect the supply leads to the main (mixer) PC board at this stage. That step comes later, after the power supply has been tested. Use cable ties to secure the mains wiring and transformer leads. Testing When the wiring is complete, go 86  Silicon Chip back over your work and check carefully for any wiring errors. In particular, check that the mains wiring is correct and that all exposed switch and fusehold­er terminals have been shrouded with heatshrink tubing. Now carry out the following test procedure: (1). Disconnect both power switch leads and the .001µF capacitor from the mains terminal block and connect an insulated link across the vacant terminals (this disconnects and bypasses the mains switch). (2). Install the fuse, apply power and check that the +15V and -15V supply rails are present on the power supply board. (3). Switch off and run the supply leads (+15V, GND, -15V) from the power supply board to the main board (immediately below the left bargraph). These leads can be run using medium-duty hookup wire. Twist the leads together to keep them neat and tidy and make sure you don’t get them mixed up; ie make sure that The power transformer and the power supply board are mounted on the base of the chassis. Make sure that the mains cord is securely anchored via its cordgrip grommet and be sure to cover all exposed terminals with heatshrink tubing. +15V goes to +15V, GND to GND, and -15V to -15V. (4). Reapply power and check that the power LED lights. If it doesn’t, check the LED orientation. Check each LM833 IC for +15V on pin 8 and -15V on pin 4. The SSM2017 and OP27GP ICs should have +15V on pin 7 and -15V on pin 4, while the LM3915s (IC6 & IC8) should have +15V on pins 3 and 9. (5). Connect a signal source to one of the inputs and check that the LED bargraph displays operate correctly. The headphone output can be checked by plugging in a set of headphones. (6). Check that the corresponding pan control shifts the signal from left to right, as indicated by the displays and the head­phone output. Do this for each of the eight main channels, then test the Auxiliary input and its pan control. (7). Check for signal at the left, right, monitor and effects outputs using an oscilloscope or a multimeter set to read AC volts. All that remains now is the final assembly. First, check that the mains plug has been pulled out of the wall TABLE 1: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value .01µF .0047µF .001µF 270pF 180pF 27pF 10pF IEC 10n 4n7 1n0 270p 180p 27p 10p EIA 103 472 102 271 181 27 10 socket, then remove the insulated link from the terminal block and reconnect the switch leads and the .001µF 2kV capacitor. TABLE 2: RESISTOR COLOUR CODES ❏ No. ❏   5 ❏ 47 ❏ 36 ❏ 46 ❏ 19 ❏ 25 ❏ 44 ❏ 10 ❏   2 ❏   7 ❏   3 ❏   2 ❏ 23 Value 68kΩ 22kΩ 15kΩ 10kΩ 6.8kΩ 4.7kΩ 2.2kΩ 330Ω 270Ω 100Ω 68Ω 33Ω 10Ω 4-Band Code (1%) blue grey orange brown red red orange brown brown green orange brown brown black orange brown blue grey red brown yellow violet red brown red red red brown orange orange brown brown red violet brown brown brown black brown brown blue grey black brown orange orange black brown brown black black brown 5-Band Code (1%) blue grey black red brown red red black red brown brown green black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown orange orange black black brown red violet black black brown brown black black black brown blue grey black gold brown orange orange black gold brown brown black black gold brown December 1996  87 PARTS LIST 1 metal case, 430 x 300 x 132mm 1 front panel (484 x 309mm) with screen printed artwork plus securing screws 1 PC board, code 01210961, 400 x 290mm 1 PC board, code 01210962, 105 x 67mm 1 20VA 2 x 15VAC toroidal transformer (T1) (Jaycar MT-2086) 1 2AG panel mount fuse holder 1 2AG fuse (F1) 8 SPDT toggle switches (S1-S8) 1 single pole 12-way rotary switch (S9) 1 SPST mains rocker switch (S10) 12 6.35mm stereo sockets (Altronics P-0075 or equiv.) 8 straight pin PC mount XLR panel sockets (Altronics P- 0883) 2 straight pin PC mount XLR panel plugs (Altronics P-0881) 29 10kΩ log pots with 38mm long shaft (VR1, VR2, VR5, VR7-10, VR12-14, VR17, VR19, VR20, VR23, VR25, VR26, VR29, VR31, VR32, VR35, VR37, VR38, VR41, VR43, VR44, VR47, VR49, VR50, VR53) 9 10kΩ lin. pots with 38mm long shaft (VR6, VR11, VR18, VR24, VR30, VR36, VR42, VR48, VR54) 8 100kΩ lin. pots with 38mm long shaft (VR3, VR15, VR21, VR27, VR33, VR39, VR45, VR51) 8 20kΩ lin. pots with 38mm long shaft (VR4, VR16, VR22, VR28, VR34, VR40, VR46, VR52) 9 red knobs 10 blue knobs 16 grey knobs 10 green knobs The front panel can now be refitted. Check that the LED displays and power LED fit their respective holes before securing the panel to the standoffs using 3mm screws. The front panel is further secured by fitting the nuts to the 6.35mm sockets and to the toggle switches, and by fitting the self-tapping screws that come with the XLR plugs and sockets. This done, the entire assembly can be fitted to the case and secured using the 88  Silicon Chip 10 black knobs 1 mains cord and plug 1 cord grip grommet 3 solder lugs 1 3-way mains terminal block 2 heatsinks, 29 x 30 x 12mm 4 6mm standoffs 10 25mm tapped standoffs 59 PC stakes 4 rubber feet 30 3mm dia. x 6mm long screws 1 3mm dia. x 9mm screw, nut and star washer 2 3mm dia. x 12mm screws and nuts 2 3mm nuts 20 self-tapping screws for XLR sockets and plugs 54 10.5mm ID x 2mm high plastic spacers for pots (Farnell 3 x 582-591 or similar) 1 6m length of 0.8mm tinned copper wire 1 300mm length of sheathed twin mains wire 1 300mm length of red hookup wire 1 300mm length of green hookup wire 1 300mm length of black hookup wire Semiconductors 8 SSM2017P balanced microphone preamplifier ICs (IC1, IC13, IC16, IC19, IC22, IC25, IC28, IC31) 8 OP27GP op amps (IC3, IC15, IC18, IC21, IC24, IC27, IC30, IC33) 14 LM833 op amps (IC2, IC4, IC5, IC7, IC9, IC10, IC11, IC14, IC17, IC20, IC23, IC26, IC29, IC32) 1 TL071 op amp (IC12) self-tapping screws supplied. It’s now simply a matter of fitting the push-on knobs to the potentio­meter and switch shafts. We suggest that red knobs be used for the Monitor controls, black for Effects and Auxiliary, red for Pan, grey for Treble, green for Bass and blue for Main. The final testing is best done using all inputs with microphones and instruments. Note that the mixer is intended to operate with the signal GND 2 LM3915 log LED bargraph drivers (IC6, IC8) 1 BC337 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 2 1N914, 1N4148 diodes (D1,D2) 4 1N4004 diodes (D3-D6) 1 15V positive regulator (REG1) 1 15V negative regulator (REG2) 4 5-segment LED bargraph displays (LEDs 1-20) (Altronics Z 0179) 1 3mm LED (LED21) Capacitors 2 1000µF 25VW PC electrolytic 2 100µF 16VW PC electrolytic 6 47µF 50V non-polarised PC electrolytic 35 10µF 35VW PC electrolytic 8 6.8µF 50V non-polarised PC electrolytic 40 2.2µF 50V non-polarised PC electrolytic 4 1µF 50V non-polarised PC electrolytic 8 .01µF MKT polyester 16 .0047µF MKT polyester 2 .001µF MKT polyester 27 270pF ceramic 2 180pF ceramic 5 27pF ceramic 8 10pF ceramic 1 .001µF 2kV ceramic Resistors (0.25W, 1%) 5 68kΩ 10 330Ω 47 22kΩ 2 270Ω 1W 36 15kΩ 7 100Ω 46 10kΩ 3 68Ω 19 6.8kΩ 2 33Ω 25 4.7kΩ 23 10Ω 44 2.2kΩ Miscellaneous Solder, heatshrink tubing. earthed at one point. This is usually done at the power amplifier. If this is not the case, then earth the signal GND to chassis at the mixer. Finally, note that the maximum input levels before clipping were incorrectly listed in the specifications panel on page 23 of the November issue. The correct levels are 260mV RMS on the Low setting and 3V RMS on the High setting (not 2.9V and 9V, SC as shown).